1. Field of the Invention
The present invention relates to a synchronizer for a communication system, or more in particular to a synchronizer in which a timing deviation can be estimated based on the average value of the difference in frame timing even in the case where a unique word (UW) pattern fails to be received, thereby making it possible to hold a frame timing even after a protracted line disconnection.
2. Description of the Related Art
A conventional synchronizer of this type, as shown in FIG. 1A, comprises a UW pattern detection circuit 2, a reference timer 5, a latch circuit 6, a histogram calculation circuit 8, a timing correction value calculation circuit 10, and a decoder 13. The UW pattern detection circuit 2 detects a unique word pattern (hereinafter referred to as "the UW pattern") contained in a known symbol pattern (normally called "the synchronous word" or "the unique word") of a receiving signal 1 shown in FIG. 1B. The reference timer 5 calculates the receiving time based on a reference clock 4. The latch circuit 6 fetches the time when the UW pattern is detected (the UW pattern detection time) by latching an output signal of the reference timer 5 by an output signal 3 of the UW pattern detection circuit 2. The histogram calculation circuit 8 calculates the histogram of the UW pattern detection time from an output signal 7 of the latch circuit 6 and further calculates the difference between the frame timing of a transmitter and the frame timing of a receiver based on the calculated histogram and an output signal 12 of the timing correction value calculation circuit 10. The timing correction value calculation circuit 10 produces on the basis of an output signal 9 of the histogram calculation circuit 8 a control signal 11 to be output to the reference timer 5 for correcting the frame timing of the receiver in the forward direction when deviated backward of the frame timing of the transmitter and backward when deviated forward of the frame timing of the transmitter. Specifically, in correcting the frame timing of the receiver in forward direction, the initial value of the reference timer 5 is set to +1 by the control signal 11, while in correcting the frame timing of the receiver backward, the initial value of the reference timer 5 is set to -1 by the control signal 11. In this way, the frame length is increased or decreased by one frame, as the case may be, to set the frame timing of the receiver to conform with that of the transmitter.
As described above, in the conventional synchronizer, the frame timing of the receiver is corrected by detecting the UW pattern constantly and thus by detecting the difference between the frame timing of the receiver and that of the transmitter. As a result, in the case where the UW pattern cannot be detected over a long time due to a line disconnection or the like, the frame timing of the receiver and that of the transmitter are deviated considerably from each other.